Light sources for free space optical communications require a substantial amount of power in order to overcome atmospherically-induced noise and the laser beam's inherent diffraction. This power requirement is greater than can be achieved with a single diode laser, although in all other aspects, diode lasers are the preferred choice as light sources because of their reliability and emission wavelength range.
One solution to achieve higher powers has been to use a pumped Nd:YAG laser. Such lasers emit continuous wave power and pulse energies which are well in excess of what is required for most communication applications, and, their wavelength (1.064 .mu.m) falls within a range where high-quantum-efficiency detectors are available.
Historically, the main problems with Nd:YAG lasers for optical communications were the low reliabilities and low efficiencies of their lamp pumps. Semiconductor diode lasers have been found to be good pump sources, especially since they can be fabricated to emit at 0.81 .mu.m, the most efficient pumping band for Nd:YAG lasers.
For the purposes of pumping, it is critical that the center frequency of the source be held constant. This differs from the requirements of optical communication and high data rate fiber transmission, where line width, rather than center frequency, is of extreme importance, with the required line width commonly being on the order of 0.00001 .ANG. to 1 .ANG.. Diode pumping is not as sensitive to line width, with an absorption band being on the order of 5 .ANG. wide, however, constant power levels are essential, and the greater the pumping power the better.
In order to achieve higher output power levels for Nd:YAG lasers, several diode lasers have been combined in arrays. These power levels are still somewhat deficient and efforts are being made toward increasing the power levels available from diode pumped Nd:YAG lasers. More pumping power is available by increasing the current, however, diode lasers are highly sensitive to wavelength shifts with temperatures. Due to the large amounts of drive current delivered to a diode pump during a single pulse, the diode heats up so quickly that the emission wavelength drifts from one extreme of the Nd:YAG laser's absorption band to the other in a matter of microseconds. Changes of this rapidity cannot be controlled by thermo-electric cooling techniques with feedback loops.
It would be desirable to provide a diode laser or an array of diode lasers which can deliver a high level of pumping power to a Nd:YAG or other solid state laser with greatly reduced sensitivity to temperature-induced wavelength shifts in the center frequency. It is to this object the present invention is directed.